A wrench is a hand or machine operated tool to apply mechanical advantage to increase torque while rotating fasteners for tightening or loosening. A closed-ended wrench may be engaged with a nut or bolt when the wrench can be passed over an end of the nut or bolt. An open-ended wrench is used where the nut or bolt can only be accessed from a side approach. Adjustable wrenches include the monkey, pipe, or crescent wrench, as well self-adjusting wrenches.